1. Field of the Invention
The present invention relates to improvements in a camera, which has a focus detecting device having, in a photographing frame, a plurality of focus detecting fields or areas that allow focus detection using outputs from some pixels of a photoelectric conversion element, an exchangeable lens device, and a camera system.
2. Related Background Art
As a conventional focus detecting device for a camera, a TTL (Through The Lens) phase difference detection type focus detecting device is known. In the TTL phase difference detecting system, object images are formed using two light beams that have passed through different portions of the pupil of a photographing lens, and the phase difference between the positions of the two object images is detected and is converted into a defocus amount of the photographing lens.
This focus detecting system is suitable for a camera system using exchangeable photographing lenses, since the amount of defocus can be detected irrespective of the focal length of the photographing lens, and the size of each focus detecting area is fixed independently of the focus length of the photographing lens. In recent years, a TTL phase difference detection type focus detecting device having a plurality of focus detecting areas also has become commercially available.
In the TTL phase difference detection type focus detecting device, the focus detecting precision suffers considerably when some light components of a focus detecting light beam are eclipsed. Hence, a photographing lens to be mounted must have a pupil size that does not eclipse any light components for each focus detecting area. For this reason, when various types of photographing lenses are applied to an automatic focus adjustment camera system, two methods may be used. In the first method, use of photographing lenses other than those that can effect all the focus detecting areas of the camera main body is inhibited. In the second method, whether or not focus detection is permitted or its result is used is determined in units of focus detecting areas in correspondence with the state of, e.g., the photographing lens.
In a system arrangement that uses the first method, an incoming focus detecting light beam on the respective focus detecting areas is free from any eclipse for all the focal lengths of all the photographing lenses to be mounted. That is, the photographing lenses do not limit any focus detecting light beams at all focal lengths.
In a system arrangement that uses the second method, an incoming focus detecting light beam on the respective focus detecting areas is likely to be eclipsed depending on the focal length of the photographing lens to be mounted. That is, the photographing lens may limit the focus detecting light beam depending on its focal length.
Photographing lenses which satisfy, e.g., the following conditions can be subjected to automatic focus adjustment in the system arrangement that uses the first method:
1) a photographing lens which has a given brightness, i.e., having a full F number that does not exceed F5.6; or PA1 2) a photographing lens having a special exit window even when it has given darkness.
Note that "the photographing lens having a special exit window" is the one which has an exit window with a position and shape at the central values of the entire photographing lens group, and for example, a reflection optical system can be used as one of such lenses.
In this automatic focus detecting camera system, any photographing lenses which are not darker than at least F5.6 can be mounted on the camera to perform automatic focus adjustment.
In the system arrangement that uses the second method, the number of photographing lenses that can be used can be increased. By contrast, since the degree of freedom on the camera side can be increased, focus detecting areas can be located over a broader range.
In a camera disclosed in Japanese Patent Publication No. 6-90349, when the largest full F number in the zooming range of a photographing lens is output to determine if focus detection of the camera is permitted, focus detection can be done using the maximum number of focus detecting areas. This is because, when a focus detecting light beam is likely to be eclipsed upon zooming of the photographing lens, focus detection can be inhibited in advance.
On the other hand, a camera disclosed in Japanese Patent Publication No. 7-62731 has two TTL focus detecting systems having different distance measurement base lengths, and the similarity between a pair of image signals is detected to select one of these focus detecting systems.
When a plurality of focus detecting areas are selectively used for focus detection depending on the nature of the photographing lens mounted or the states of image signals, since the imaging position of the photographing lens can be controlled on the basis of focus detecting information from only focus detecting areas, the focus detecting precision of which is optically guaranteed, an accurately focused photograph can be taken.
Among such known system arrangements, assume a case wherein a teleconversion lens for extending the focal length of a photographing lens is additionally mounted. For example, in the case examined below, a photographing lens having a focal length=300 mm and full F number=4 is used as a master lens, a rear converter optical system for doubling the focal length is used as a teleconversion lens, and a camera having a total of five focus detecting areas at the center, and right and left positions of the frame is used.
At this time, a photographing lens system as a combination of the master lens and teleconversion lens has "focal length=600 mm/full F number=8". As disclosed in Japanese Patent Publication No. 6-19488, a system, which comprises a lens drive means in a teleconversion lens for increasing the focal length of a photographing lens, and uses a master lens dedicated to manual focusing, can be considered a typical example of such lens system.
The master lens is capable of automatic focus adjustment in either system arrangement using the first or second method, since the full F number of its specifications "300 mm/F4" is brighter than F5.6.
However, the full F number of the photographing lens system including the teleconversion lens is F8, which is darker than F5.6. Since this lens system is not a photographing lens having a special exit window, the system arrangement that uses the first method cannot use this photographing lens system as an automatic focus adjustment system. That is, manual focusing must be done when this system is used.
On the other hand, since the system arrangement that uses the second method determines in units of focus detecting areas depending on the state of the photographing lens and the like whether or not focus detection is permitted or its detection result is used, three focus detecting areas near the optical axis, i.e., the center of the photographing lens may be used in focus detection.
That is, as explained in Japanese Patent Application Laid-open No. 5-323182, whether or not a focus detecting light beam is likely to be eclipsed by focus detecting areas at off-axis positions is determined by the relationship between each focus detecting area and the exit window of the photographing lens system corresponding to that focus detecting area position. If eclipse takes place, the pair of image signals obtained have poor similarity in many cases, and the focus detection result cannot be used in such case. Therefore, if eclipse is predicted, focus detection of the corresponding focus detecting areas must be inhibited.